In a standardized balance sheet every standard line item is named, i.e. it is retrieved using a specific field name. The example in my previous response uses field names TR.F.BVExclOthEq and TR.F.SecHeldUnderCollat. You can group these fields (and all other named fields you'd like to retrieve) any way you like. E.g. you could use one get_data call to retrieve all fields that fall under assets and another call to retrieve all fields that fall under liabilities. Or you could create the list of fields where the first X fields fall under assets and the rest fall under liabilities.

The best way to explore the metadata for use with Eikon Data APIs is through CodeCreator app, which among other things allows you to search for field names or browse fields by category, see what parameters are applicable to selected field etc. And at the end it produces a code snippet, which you can copy & paste into whatever IDE you're using.

I'm not sure I quite understand the use case here, but it sounds to me like you're looking for a standardized balance sheet instead of "as reported" one, i.e. a balance sheet with standardized line items. If this is what you're looking for, create a list of the line items you're interested in and use them in the fields argument of get_data method, e.g.

df, err = ek.get_data(['MS.LM'], 
                      ['TR.F.BVExclOthEq',
                       'TR.F.SecHeldUnderCollat',
                       'TR.F.PeriodEndDate'], 
                      parameters = p )